Device for preparation of plastic materials for recycling purposes

ABSTRACT

A device for the preparation of plastics material for recycling purposes has a container ( 1 ) with an upper inlet opening ( 10 ) for the material. In the container ( 1 ), at least one tool ( 14 ) rotates about the vertical container axis. This tool feeds the treated material through an outlet opening ( 17 ) which is arranged at the bottom of the container ( 1 ) and to which the housing ( 19 ) of at least one screw ( 21, 22 ) is connected. That side wall ( 28 ) of the feed opening ( 18 ) at which the tool ( 14 ) discharges defines a widening ( 36 ) extending continuously towards the screw axis. This widening ( 36 ) extends over part of the circumference of the screw turns ( 30 ) so that the screw-side edge of this widening ( 36 ) forms an uninterrupted shearing edge ( 39 ) co-operating with the screw turns ( 30 ).

The invention relates to a device for the preparation of plastics material for recycling purposes, in particular thermoplastic plastics material, comprising a container into which the material to be processed is fed from above through an inlet opening, and at least one mixing and/or comminuting tool arranged in the container and rotating about the vertical axis of the container, wherein the material treated by this tool is fed through an outlet opening which is arranged at the bottom of the container and to which the feed opening of a housing of at least one screw is fluidically connected, the housing being connected, preferably tangentially, to the container, and wherein that side wall of the feed opening at which the tool discharges, seen in section, defines a widening of the feed opening, the widening extending continuously towards the axis of the housing of the screw.

Devices of this type are known, e.g. AT 407 972 B. They usually have an extruder screw which feeds the material conveyed by it directly to the following processing stage, usually a granulator. However, the screw can also be a simple conveyor screw which feeds the material to a following processing step, e.g. further processing by means of rotating tools. Only after this processing step is the material fed to a plasticiser or agglomerator. Essentially, the known devices operate satisfactorily, but frequently have difficulties when charging the apparatus connected to the outlet opening, in particular when this apparatus is a plasticiser or agglomerator. The infeed to the screw should be as uniform as possible and the delivery from this apparatus should also be as uniform as possible, which is of particular importance when a moulding plant is connected to an extruder. In the case of a moulding plant, it is important that the material to be moulded is supplied as uniformly as possible. The aforementioned difficulties are caused, among other things, by the fact that the material to be processed usually varies greatly, both with respect to the chemical composition of the material and with respect to its external constitution (degree of soiling, particle size, particle shape, etc.).

The object of the invention is to overcome these difficulties and improve a device of the initially described type in such a way that the material to be processed is fed as uniformly as possible into the feed opening of the screw housing, the feed opening being connected to the outlet opening, so that a uniform output is achieved at the delivery end of the screw, in particular of a plasticiser or agglomerator. The invention achieves this object in that the widening, seen in the direction of the axis of the housing of the screw, extends over part of the circumference of the turns of the adjacent screw, preferably into the region of the screw axis, so that the screw-side edge of this widening forms an uninterrupted shearing edge co-operating with these screw turns. This produces a constant infeed of the material to be processed into the feed opening of the screw, even when the material in the container is soft through heating. It may be assumed that this improved mode of operation is attributable to the comparatively long shearing edge formed by the screw-side edge of the widening. A further essential advantage of the design according to the invention lies in the fact that the cooled grooved sleeve, which has hitherto usually been necessary in the infeed region and which is subjected to considerable wear, is no longer required. This not only represents a cost saving, but also brings with it the advantage of a lower starting torque for the drive motor.

According to a particularly advantageous further development of the invention, the side wall defining the widening of the feed opening forms an angle of 20° to 80°, preferably 30° to 55°, with the screw axis. Experiments have also shown that it is particularly advantageous for the aforementioned infeed behaviour if the following formulae apply to the feed opening of the housing of the screws: h≧k0.6d and L≧k0.6d, where

-   -   h is the effective height of the feed opening in mm, measured in         the axial direction of the container,     -   L is the effective length of the feed opening in mm, measured in         the circumferential direction of the container,     -   d is the outer diameter of the screws in mm and     -   k is the number of screws, k preferably being equal to 1 or 2.

Furthermore, it is advantageous if the base of the feed opening slopes downwards from the screws towards the container.

The invention provides advantages both in designs in which only a single screw is arranged in the housing connected to the outlet opening of the container, and in designs in which two screws are mounted in this housing. This screw or these screws can be plasticising or agglomerating screws, but also simple conveyor screws. In twin-screw designs, the relatively high costs previously had to be considered, but the applicants' experience has shown that the complicated measures which were necessary in single-screw designs in order to achieve some degree of uniform infeed behaviour come close to the cost of a twin-screw design. In addition, the screws and barrels of twin-screw designs can be substantially shorter than in single-screw designs, and approximately 1.5 to 2 times the output is achieved with the same screw diameter (e.g. in polyethylene processing).

However, the applicants' experiments have shown that, in twin-screw designs, in order to maintain the advantages of the infeed behaviour, it is advantageous if the diameter D of the container and the diameter d of each of the two screws are in the following relationship: ${D = {10\sqrt[3]{d^{2}K}}},$ where

-   -   D is the inner diameter of the circular cylindrical container in         mm or the inner diameter of a hypothetical circular cylindrical         container converted to the same capacity and having the same         effective height,     -   d is the screw diameter in mm and     -   K is a constant greater than or equal to 50, preferably greater         than 100.

A conversion has to be made for containers which are not circular cylindrical and screw diameters which are not constant, which will be discussed in further detail later.

These twin-screw designs make it possible to keep the screw length shorter than in single-screw designs, shorter by approximately half, so that substantially less space is required for the same output of the device. Better plasticising properties are produced in the wedge region of the two screws because there the material is particularly intensively worked by shearing and kneading. It has also been shown, with respect to the screw geometry, that this type of design is less sensitive to the processing of different types of plastics, with the result that the device is more universally usable.

It has also been shown, within the framework of the invention, that the charging of the apparatus formed by the two screws and their housing, in particular if it is formed by a plasticiser or agglomerator, improves as the surface contacted by the material is reduced, the surface for the material to be processed lying between the container and the screws. Therefore, according to a further development of the invention, the design is such that, with two screws in the housing, the screws lie as closely as possible to the container-side edge of the feed opening. The housing of the plasticiser or agglomerator formed by the screws is therefore directly connected to the outlet opening of the container so that the feed opening of the housing of the plasticiser or agglomerator is directly adjacent to the outlet opening of the container or coincides therewith. For this purpose, it is advantageous if the housing of the two screws is connected as tangentially as possible to the circumference of the container, although different arrangements are in principle also possible, e.g. a radial arrangement.

The most favourable arrangement within the framework of the invention is produced when the cross-sections of the two screws in the housing of the plasticiser or agglomerator lie vertically or obliquely above one another in the region of the feed opening. This vertical arrangement produces the smallest surface that the material to be processed, which from time to time can be sticky, has to cross from the container to the two screws. This is important in order to prevent the material from adhering to these surface portions. If there is a departure from the vertical arrangement, e.g. with an oblique arrangement of the two screw cross-sections or even a side-by-side arrangement, it is then usually necessary to provide cooling ducts in the housing of the plasticiser or agglomerator in the region of the feed opening in order to prevent the processed material from adhering to these surface portions and to prevent the resultant overheating of the accumulated material.

The turns of the two screws can mesh with one another at least in the region of the feed opening. This has the advantage of self-cleaning of the screw turns, which is particularly important in the infeed region. Although equal diameters of the two screws are advantageous, this is not absolutely necessary, and the two screws can also have different lengths, e.g. when one screw acts as a feed screw for the other screw.

Further features and advantages of the invention will become apparent from the description of embodiments of the subject of the invention, which are schematically shown in the drawings. FIG. 1 shows a vertical section through a first embodiment, seen along the cutting line I-I in FIG. 2, which shows a section along the line II-II in FIG. 1. FIG. 3 shows an embodiment in horizontal section with two screws of equal length lying side by side. FIG. 4 shows a variation of the embodiment in FIG. 3, in which the two screws have different lengths. FIG. 5 shows the formation of the feed opening in an arrangement with only one screw. FIG. 6 shows the formation of the feed opening in an arrangement with two screws, and FIG. 7 shows the formation of the feed opening according to FIG. 5 in horizontal section. FIGS. 8 to 14 show different possibilities for the arrangement of the screws in twin-screw designs, in each case in vertical section. FIG. 15 shows a horizontal section through an arrangement with a conical twin-screw extruder.

The device according to FIGS. 1, 2 and 6 is a shredder/extruder combination with a twin screw and has a container 1 with a vertical axis 2 and a circular cross-section, the side wall 3 of which extends substantially cylindrically. A carrier plate 4 rotates about the axis 2 in the container 1 in the direction of the arrow 8 (FIG. 2) and sits on a shaft 5 which extends through the base 6 of the container 1 in a sealed manner and is rotatably mounted in the base 6 in bearings 7. The shaft 5 is rotatably driven by a motor 9.

The top of the container 1 has an inlet opening 10 for the material to be processed, advantageously in a funnel-shaped covering wall 11. If the material is to be processed under vacuum, an evacuator is connected to the container 1, and the inlet opening 10 is provided with a sluice which can advantageously also be evacuated. The material to be processed is fed to the inlet opening 10 by a feeder 12, e.g. a conveyor belt, and dropped into the opening 10 in the direction of the arrow 13.

The carrier plate 4, arranged in the region of the base 6, carries a plurality of tools 14 which at least have a mixing effect on the material introduced into the container 1, usually thermoplastic material for recycling, and usually also comminute and dry the material. In the latter case, the tools 14 are formed as cutters with blades. The material spun off from the tools 14 of the carrier plate 4 rises up the side wall 3 of the container 1 in the form of a mixing vortex 15 and, after reaching a culmination point 16, falls back into the region of the container axis 2. In this way, the processed material is thoroughly mixed or comminuted and dried and, after an adequate residence time in the container 1, is discharged from the latter through an outlet opening 17 (FIG. 2) which, in the embodiment shown, coincides with the feed opening 18 of the housing 19 of a twin-screw arrangement of a plasticiser or agglomerator 20, e.g. an extruder, but is at least fluidically connected to the feed opening 18. The apparatus 20 has two screws 21, 22 which are arranged in the housing 19 and which are synchronously driven in the same direction of rotation by a motor 23 so that their screw turns can mesh with one another. A possible drive arrangement suitable for this is shown in FIGS. 3 and 4. The motor 23 drives a sun gear 24 which is mounted in a transmission housing 25 and which drives two planet gears 26, 27 connected in a rotationally fixed manner to the shafts of the two screws 21, 22.

To obtain double the output quantity in comparison with a single-screw extruder, it has proved advantageous if the container dimensions satisfy the relationship according to the following, empirically determined rule of thumb: ${D = {10\sqrt[3]{d^{2}K}}},$ where D is the inner diameter of the container 1 in mm, d is the outer diameter of the turns of the two screws 21, 22 in mm and K is a constant which is at least 50, but preferably at least 100. This formula applies to a circular cylindrical container. If this container is not circular cylindrical or not cylindrical, e.g. conical, D has to be converted to the inner diameter of a hypothetical circular cylindrical container of the same capacity, i.e. the same effective height. The effective height H is regarded as the distance of the culmination point 16 of the mixing vortex 15 above the upper edge of the carrier plate 4, provided that the operating conditions are suitable.

In the case of different screw diameters, the average value of the screw diameters is to be taken for d in the above formula. If the screw diameter is not constant over the length of the screw (e.g. in conical screws), then the average screw diameter is to be used.

As already mentioned, the size and shape of the feed opening 18 of the screw housing 19 are also important. The plastics material which is mixed in the container 1, optionally comminuted, and thereby heated and optionally dried, is cast by the tools 14 into this feed opening 18, which lies at least in part at the level of the tools 14 rotating in the container 1. This can be assisted by the leading edges of the tools 14—seen in arrow direction 8—being angled or bent counter to the direction of movement so that a spatula-type pressing action of the material into the feed opening 18 takes place during rotation of the tools 14. Experiments have shown that conditions are particularly favourable produced if the following measurements apply to the feed opening 18: h≧k0.6d and L≧k0.6d, where

-   -   h is the effective height of the feed opening 18 in mm, measured         in the axial direction of the container 1,     -   L is the effective length of the feed opening 18 in mm, measured         in the circumferential direction of the container 1,     -   d is the outer diameter of the screw turns in mm and     -   k is the number of screws, k preferably being 1 or 2.

The effective clear cross-section in relation to the direction of passage of the material from the container 1 into the housing 9 counts as the effective length or effective height of the feed opening 18. For screw diameters which are not constant, d has to be converted, as already mentioned.

Furthermore, experiments have shown that the material introduced into the feed opening 18 has the tendency from time to time to accumulate in the region of that edge of the feed opening 18 at which the tools 14 discharge, i.e. the left-hand edge of the feed opening 18 in FIG. 2. In order to avoid this accumulation, it has proved advantageous to arrange that side wall 28 of the feed opening 18 at which the aforementioned tools 14 discharge (this is the side wall 28 lying on the right in FIGS. 3 and 4) obliquely in relation to the longitudinal axis 29 of the screw housing 19 so that, seen in the infeed direction of the plastics material, a widening 36 of the feed opening 18 is formed which extends continuously towards the screw axis. The angle α (FIG. 7), which this side wall 28 forms with the longitudinal axis 29 of the screw housing 19, is in any case less than 90°, especially 20° to 80°, preferably 30° to 55°. The side wall 28 extends at least as far as the circumference of the screw turns 30 of the adjacent screw 21 or 22, preferably into the region of the axis of this screw. With a vertical arrangement of the two screws 21, 22, seen in plan view, this longitudinal axis of the screw coincides with the axis 29 of the screw housing 9 (FIG. 2). In single-screw designs (FIGS. 5, 7), the side wall 28 extends at least over part of the circumference of the turns 13 of the screw, preferably into the region of the screw axis. In all cases, the edge of this side wall 28, in particular its end, forms a shearing edge 39 (FIGS. 1, 5, 6) which co-operates with the screw turns 30 and which, because a grooved sleeve is not provided, extends continuously, i.e. without interruption, in the circumferential direction of the screws 21, 22.

In the embodiment according to FIGS. 1, 2 and 7, the two screws 21, 22 are arranged vertically above one another, and the screw axes lie horizontally. The two are not absolutely necessary, although these arrangements are the most favourable. If there has to be a departure from this, the two screws 21, 22 can lie horizontally side by side, as FIGS. 3 and 4 show. In the embodiment according to FIG. 3, the two screws 21, 22 are of equal length and driven in the same direction of rotation; their screw turns 30 can mesh with one another. The two screws 21, 22 therefore feed jointly into an extruder head 31 connected to the screw housing 19.

The arrangement according to FIG. 4 differs from that according to FIG. 3 in that the two screws 21, 22 have different lengths, namely the screw 21 adjacent to the container 1 is shorter than the other screw 22. The screw 21 therefore takes up the material cast out of the container 1 through its outlet opening 17 and into the feed opening 18 of the screw housing 19, this infeed behaviour being improved by the aforementioned dimensioning of the container 1 and the feed opening 18 and also by the formation of the side wall 28 of the same. During its conveying action, the screw 21 transfers the material taken up by it to the other screw 22, which feeds the material to the connected extruder head 31. Apart from the advantage of a uniform infeed, this arrangement has the further advantages of a cheaper delivery housing 37 and a cheaper inlet piece leading to the extruder head 31.

FIG. 5 shows in more detail the formation of the feed opening 18 for designs according to FIGS. 3 and 4.

It can be seen from FIGS. 5, 6 and 7 that a chamfer 32 is also provided on that edge of the feed opening 18 at which the tools 14 discharge, i.e. the right-hand edge in FIGS. 5, 6 and 7, in order to improve the infeed behaviour. Furthermore, the base 33 of the feed opening 18 is inclined towards the container 1 so that the wall—formed by the base 33—of the feed opening slopes downwards from the screw 22 towards the container 1.

As already mentioned, the two screws 21, 22 can in principle adopt any position relative to one another. Various arrangements are shown in FIGS. 8 to 14. The position of the two screws 21, 22 vertically above one another as shown in FIG. 11 is the most advantageous arrangement because then there is no risk of plastics particles adhering to the surface of the infeed region of the screw housing, unlike in other positions of the screws 21, 22. Arrangements deviating from the arrangement according to FIG. 11 are in principle also usable, but in these arrangements the aforementioned adhesion of e.g. sticky plastics particles and the resultant blockage of the charging opening are possible. This can lead to overheating of these material particles. In order to avoid this, it is recommended, for arrangements of the screws 21, 22 deviating from FIG. 11, to provide cooling ducts 34 in the region of that portion of the housing 19 which is defined by the feed opening 18, where cavities 35 are located adjacent to the feed opening 18, which cavities 35 can lead to the aforementioned adhesion and the resulting excessively long residence times of the worked material particles. The larger these cavities 35 are and the further they extend into the housing 19 away from the container 1, the more cooling ducts 34 necessarily have to be provided.

Although the formation of the two screws 21, 22 with an equal and constant turn diameter d (FIG. 7) is the most favourable on grounds of cost, this is not absolutely necessary. FIG. 15 shows an embodiment with two screws 21, 22 which are both conically formed and together form a twin-screw extruder which feeds into an extruder head 31. The two screws 21, 22 are driven in opposite directions by a common motor 23 via mutually meshing gears 38. Their screw turns 30 can mesh with one another so that self-cleaning in the infeed region is also achieved in this embodiment.

Although the horizontal arrangement of the housing 19 having one or two screws is the most favourable, it is possible in principle to connect this housing with an inclined longitudinal axis to the container 1, if necessary even vertically.

Although, in the above-described embodiments, only two screws are shown in each case, designs with three or more screws are theoretically also possible in an analogous construction. The aforementioned formulae also apply to these designs. For economic reasons, however, designs of this type provided with three or more screws are not of great significance. 

1. A device for the preparation of plastics material for recycling purposes, in particular thermoplastic plastics material, comprising a container (1) into which the material to be processed is fed from above through an inlet opening (10), and at least one mixing and/or comminuting tool (14) arranged in the container (1) and rotating about the vertical axis (2) of the container (1), wherein the material treated by this tool (14) is fed through an outlet opening (17) which is arranged at the bottom of the container (1) and to which the feed opening (18) of a screw housing (19) is fluidly connected, the screw housing (19) being tangentially connected to the container (1), and wherein that side wall (28) of the feed opening (18) at which the tool (14) discharges, seen in section, defines a widening (36) of the feed opening (18), the widening (36) extending continuously towards the axis (29) of the screw housing (19), characterised in that this widening (36), seen in the direction of the axis (29) of the housing (19) of two or more screws (21, 22), extends over part of the circumference of the turns (30) of the adjacent screw (21, 22), preferably into the region of the screw axis, so that the screw-side edge of this widening (36) forms an uninterrupted shearing edge (39) co-operating with these screw turns (30).
 2. A device according to claim 1, characterised in that the side wall (28) defining the widening (36) of the feed opening (18) forms an angle between 20° and 80°, preferably between 30° and 55°, with the screw axis.
 3. A device according to claim 1, characterised in that the following formulae apply to the feed opening (18) of the housing (19) of the screw(s) (21, 22): h≧k0.6d and L≧k0.6d, where h is the effective height of the feed opening (18) in mm, measured in the axial direction of the container (1), L is the effective length of the feed opening (18) in mm, measured in the circumferential direction of the container (1), d is the outer diameter of the screw (21) or screws (21, 22) in mm and k is the number of screws, k preferably being
 2. 4. A device according to claim 1, characterised in that the base (33) of the feed opening (18) slopes downwards from the screw (21, 22) towards the container (1).
 5. A device according to claim 1, characterised in that, with two screws (21, 22) in the housing (19), the screws (21, 22) lie as closely as possible to the container-side edge of the feed opening (18).
 6. A device according to claim 1, characterised in that, with two screws (21, 22), cooling ducts (34) are provided in the wall portion of the housing (19) adjacent to the screws (21, 22).
 7. A device according to claim 1, characterised in that, with two screws (21, 22), the diameter (D) of the container (1) and the diameter (d) of each of the two screws are in the following relationship: ${D = {10\sqrt[3]{d^{2}K}}},$ where D is the inner diameter of the circular cylindrical container (1) in mm or the inner diameter of a hypothetical circular cylindrical container converted to the same capacity and having the same effective height, d is the screw diameter in mm and K is a constant greater than or equal to 50, preferably greater than
 100. 8. A device according to claim 1, characterised in that, with two screws (21, 22), the cross-sections of the two screws (21, 22) lie vertically or obliquely above one another in the region of the feed opening (18).
 9. A device according to claim 1, characterised in that, with two screws (21, 22), the turns (30) of the two screws (21, 22) mesh with one another at least in the region of the feed opening (18). 